Cross-Stream Stereo PIV Measurements of Turbulent Boundary Layers Overlying Heterogeneous, Oblique Surface Roughness

Streamwise-elongated, spanwise heterogeneous surface roughness can give rise to sustained turbulent secondary flows that can alter mass and momentum transfer relative to smooth-wall and less heterogeneous rough-wall flow. This contribution explores the persistence of these roughness-induced turbulent secondary flows when the mean flow is rotated away from the streamwise-aligned nature of such surfaces. This obliquity reflects the possible varying orientation of the mean flow relative to roughness in practical applications. Streamwise-aligned ridges spaced periodically in the spanwise direction are utilized as the baseline roughness model and this topography is rotated relative to the streamwise direction to introduce obliquity. Models of these roughness scenarios were fabricated, including a transparent roughness tile for each scenario deployed at the measurement position that was cast in urethane to match its refractive index to that of the working fluid (sodium iodide) of the flow facility which yielded an optical continuum between the roughness and the surrounding fluid. High-frame-rate stereo PIV was deployed in the cross-stream plane (wall-normal by spanwise) to explore the impact of obliquity on the turbulent secondary flows and inner-outer interactions.